Acoustic sensor

ABSTRACT

Disclosed are devices, systems, and methods for capturing acoustic data in an efficient manner. Some embodiments have piezoelectric sensing portions with polarization axes and conducting layers. In some embodiments, piezoelectric sensing portions can be positioned generally coplanar to each other and in partial electrical contact. In some embodiments, polarization axes of two piezoelectric sensing portions have a non-zero angle between them.

PRIORITY INFORMATION

This application claims priority to U.S. Patent Provisional ApplicationNo. 60/692,515, titled “ACOUSTIC SENSOR,” filed Jun. 21, 2005, theentirety of which is hereby incorporated by reference and made part ofthis specification.

BACKGROUND OF THE INVENTIONS

1. Field of the Inventions

The inventions described herein relate generally to the field oftransducers, and in particular acoustic transducers. For example, someembodiments relate to acoustic sensors that can detect biological soundsand generate accurate data for signal processing to determine biologicalcharacteristics relating to the source of those sounds.

2. Description of the Related Art

Transducers are the operative portion of many modem technologies. Oneuseful class of transducers converts an analog signal, such as anacoustic vibration wave, into an electrical signal. In particular,microphones contain acoustic transducers and can detect and recordsignals that correspond to sounds. The human hear is itself an acoustictransducer.

Designers of acoustic sensors are continually challenged by the problemof separating the desired signal from unwanted noise. This challengeapplies to both the acoustic noise (or extraneous acoustic vibrations)as well as the electronic noise (or unwanted electrical signals).Acoustic noise can be distracting background chatter that would bedetectable by a human ear, or minute, unheard vibrations caused by adistant truck driving down the street. This kind of noise can interferewith the input of an acoustic sensor. Electronic noise can beelectromagnetic emissions that cause the electrons in an electricaldevice to vibrate or move. This kind of noise can interfere with theoutput of an acoustic sensor. Because a transducer changes one signal toanother signal, it is subject to problems with noise for both types ofsignals.

Another problem that occurs in current sensors is over-sensitivity tothe direction of the signal. For example, in many cases, sensors arestructurally capable of effectively detecting signals, but are toosensitive to the orientation of the sensor with respect to the signal.Even relatively small changes in the orientation of the sensor cansignificantly affect the strength of the received signal, or determinewhether the signal is received at all. Thus, many sensors areinefficient because they depend too much on proper orientation. This canlead to repeated tests (if the error is perceived by the operator), orincorrect and unreliable readings.

Another problem of existing sensors relates to the arrival of a signalat various portions of the sensor at different times. For example, insome sensors that have multiple sensing portions that are verticallystacked, one above another, signals arriving from below the stack reachone sensing portion at one time, but that same signal does not reach theother sensing portion until later. This time difference of arrival cancreate signal time incidence ambiguities in sensor output.

Thus, there is a need for methods and devices for increasing thesensitivity of acoustic transducers, improving shielding, reducingunwanted noise, and enhancing signal to noise ratios. There is also aneed for methods and devices for improving the ability of acousticsensors to receive signals from various directions without requiringtime-consuming and error-prone repositioning of the sensors. Moreover, aneed exists for improving sensors to minimize problems with the timedifference of arrival at various sensing elements and to minimize signaltime incidence ambiguities in signal sensor outputs.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the inventions will now be briefly described withreference to the drawings. These drawings are examples and theinventions are not limited to the subject matter shown or described.

FIG. 1 is a schematic, perspective view of a sensing layer component ofa sensor in accordance with one embodiment of the inventions.

FIG. 2 is a schematic, cross-sectional side view of two sensing layercomponents, taken along the lines 2-2 of FIG. 3.

FIG. 3 is a schematic plan view of the sensing layer components of FIG.2 with electrical leads and other components.

FIG. 4 is a schematic, partial cross-sectional side view (taken alonglines 4-4 of FIG. 5) of a portion of a sensor in accordance with oneembodiment of the inventions.

FIG. 5 is a schematic perspective view of a sensor in accordance withone embodiment of the inventions.

FIG. 6 is a schematic illustration of multiple sensors positioned on thesurface of a patient's chest with electrical leads transmitting data toa processor.

FIGS. 7A-7C are schematic illustrations of certain concepts relating topiezoelectric polarity and electric charges induced by bending ofpieozoelectric materials.

FIGS. 8A-8B are schematic illustrations of multi-dimensional bending ofplanar materials and corresponding vector principles.

FIG. 9A is a schematic, cross-sectional illustration of a cut-away sideview of a sensor in accordance with one embodiment of the inventionspositioned on the skin of a patient, and a point source emittingsubstantially spherical sound waves.

FIG. 9B is a schematic, cross-sectional illustration of the sensor,point source, and sound waves of FIG. 9A at a later instant in time.

FIG. 9C is a schematic, three-dimensional, elevational illustration ofthe sensor, point source, and sound waves of FIG. 9B.

FIG. 10 is a perspective view of one alternative embodiment of a sensorin accordance with the inventions.

FIG. 11A is a perspective view of another alternative embodiment of asensor in accordance with the inventions.

FIG. 11B is a plan view of the sensor of FIG. 11A.

FIG. 11C is a schematic, cross-sectional side view of the sensor of FIG.11A, taken along the lines 11C-11C of FIG. 11B.

FIG. 12A is a schematic electronic circuit diagram illustrating theeffective electrical properties of one embodiment of a sensor with twosensing layers.

FIG. 12B is a schematic electronic circuit diagram illustratingelectrical apparatus that can be attached to the circuit of FIG. 12A fortesting and/or data processing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a sensing layer 100 comprises a piezoelectriccentral portion 114 with a top conductive layer 110 and a bottomconductive layer 112. The piezoelectric material can be a piezoelectricco-polymer. In one preferred embodiment, the piezoelectric portion 114is formed from polyvinylidene fluoride (PVDF). PVDF is an anisotropicpiezoelectric polymer that produces surface charges of substantiallyequal magnitude and opposite polarity on opposite surfaces when amechanical strain is imposed on the material. Other preferred materialsthat may form the sensing portions can comprise compounds that includevinylidene fluoride. One preferred material is a piezoelectricco-polymer that is 75% vinylidene fluoride by weight. Metallized PVDFcan be obtained from Measurement Specialties, Inc., of Hampton Va.23666. PVDF can be used to good effect due to its compliance andresilience, as well as its piezoelectric properties. Moreover, PVDF isinexpensive, is the most commonly used commercial piezoelectric polymer,and has properties which are relatively unaffected by synthesisconditions. Other piezoelectric materials can also be used, such as polyvinylidene cyanide and its copolymers; aromatic and aliphatic polyureas;poly vinyl chloride; aromatic polyamides (odd nylons); PVDF copolymerswith trifluoroethylene (P[VDF-TrFE)), tetrafluoroethylene (P[VDF-TFE)),and hexafluoropropylene (P[VDF-HFP)); PVDF blends with poly methylmethacrylate (PMMA); poly vinyl fluoride; poly vinyl acetate; andferroelectric liquid crystal polymers.

As illustrated schematically, the piezoelectric portion 114 is notcompletely surrounded by the metallized portions (top conductive layer110 and bottom conductive layer 112). Preferably, the top conductivelayer 110 and the bottom conductive layer 112 are not in electricalcontact with each other when the sensing layer is in the illustratedconfiguration. Neither the sensing layer 100, nor its sub-layers (thetop and bottom conductive layers 110 and 112 and the piezoelectricportion 114) are shown to scale in FIG. 1.

The conducting layers 110 and 112 can comprise metallization layers thatare adhered to the surfaces of the piezoelectric portion 114. Theconducting layers 110, 112 can adhere to the surface of thepiezoelectric portion by any suitable process, such as a depositionprocess. Metallization of the surfaces of the piezoelectric portion 114may be accomplished using any suitable material and any suitabletechnique known in the art. For example, thin layers of a metal, such asnickel, silver, copper or alloys thereof, can be deposited on the innerand outer surfaces of the sensing layer 114. In other embodiments, theconductive layers 110 and 112 can comprise or be coated with aconducting ink.

In a preferred embodiment, the piezoelectric portion 114 is preferablythicker than either of the two conducting layers 110, 112. In someembodiments, the piezoelectric portion 114 has a thickness of about 100microns or less. In certain preferred embodiments, the piezoelectricportion 114 has a thickness of less than 150 microns, and the top andbottom conducting layers 110 and 112 each have a thickness of less than30 microns. In one preferred configuration, the sensing layer 100comprises PVDF with a copper nickel alloy coating. The piezoelectricportion can have a thickness between approximately 150 μm andapproximately 6 μm. In some preferred embodiments, the total thicknessof the piezoelectric portion 114 and the two conducting layers 110 and112 combined is approximately 28 μm.

Typically, when a tensile strain is imposed on the piezoelectric portion114, one surface of the piezoelectric portion 114 acquires a positivecharge relative to the other. The charge is typically transferred to oneof the adjacent conductive layers 110 or 112. The piezoelectric portion114 is advantageously polarized such that the piezoelectric effect isgreater when the piezoelectric portion is stretched in a particulardirection. The polarization axis can also be referred to as the “stretchaxis.” Although biaxial orientation (stretching in two in-planedimensions) is possible, it produces piezoelectric films with lowerbilaterally isotropic piezoelectric properties. Most commerciallyavailable PVDF is uniaxially drawn, providing a high level ofpiezoelectric response along the stretch axis, or axis of orientation.

When the piezoelectric portion 114 is under strain, the oppositelypolarized charge that accumulates on the opposite layers of thepiezoelectric portion spreads out over the top and bottom conductivelayers 110 and 112, forming a capacitative effect between the twoconductive layers 110 and 112. Because of this configuration, thevoltage as measured across the two conductive layers 110 and 112 isrelated through the capacitance equation: Q=CV, where Q is the amount ofsurface charge, C is the capacitance, and V is the voltage output. Q canbe expressed in Coulombs, C can be expressed in Farads, and V can beexpressed in Volts. Certain configurations of PVDF materials exhibit apredictable voltage output V in response to a specific applied force.Generally, the amount of surface charge Q is proportional to the strainon the piezoelectric material, and capacitance C is substantiallyconstant for a given material and structure. Thus, both Q and V aregenerally proportional to the strain on the piezoelectric material. Ifthe voltage or charge response function is known, a measurement ofeither parameter can provide information about the strength of thesignal (e.g., acoustic vibration) causing the strain. Moreover, if theprecise response function of the piezoelectric material for a givenphysical configuration is not known, the output voltage can stillprovide useful data because the responses at various times can becompared.

Furthermore, if the piezoelectric portion is polarized, informationrelating to the direction of the acoustic energy can also be obtained.Alternatively, a combination of two piezoelectric portions that arepolarized in different directions can be configured to provide accuratedata regarding the magnitude of the sensed signal, independent of thesignal direction upon arrival at the sensor.

With reference to FIG. 2, two sensing layers 210 and 220 (each similarto the sensing layer 100 described above) are positioned in a generallyparallel, partially displaced, and generally coplanar orientation. Whilethe two sensing layers 210 and 220 are not precisely coplanar in theillustrated embodiment, they are only vertically shifted by the width“d” of a single sensing portion, which can be less than 100 microns, asdiscussed above. In FIG. 2, the two sensing layers 210 and 220 areschematically illustrated in cross section. The sensing layer 210 has atop conductive layer 212 and a bottom conductive layer 214. The sensinglayer 220 has a top conductive layer 222 and a bottom conductive layer224. In a preferred configuration, the conductive layers 214 and 222 arein electrical contact and are configured to receive charges of the samepolarity from their respective piezoelectric portions (the layer 214receives charge when the layer 216 is appropriately stressed, and thelayer 222 receives charge from the layer 221 under the appropriatestress). For example, if the bottom conductive layer 214 accumulatesnegative charge when the piezoelectric portion 216 is under strain, thetop conductive layer 222 accumulates negative charge when thepiezoelectric portion 226 is under strain. Similarly, the outer twoconductive layers, 212 and 224, accumulate charge of the same polarity.

The described configuration, where the sensing layers 210 and 220 areinverted with respect to each other (that is, configured to have chargeof opposite polarity accumulate on the top and bottom layers of the twosensing layers, respectively), provides the advantage of allowing asingle electrical lead to contact two conductive layers. (The electricallead 244 is in contact with both the conductive layers 214 and 222. SeeFIG. 3).

With reference to FIG. 3, a plan view of the two sensing layers 210 and220 is illustrated schematically. In this figure, the polarization ofthe two sensing layers 210 and 220 is shown with the two-sided arrowsand the letters “P.” Thus, the sensing layer 210 is polarizedsubstantially orthogonally to the sensing layer 220. The orthogonallypolarized sensing portions 210 and 220 provide a multi-directionalsensing capability. For example, a signal that bends the sensing portion210 in such a way that little or no electrical response is produced inthat portion will have a higher likelihood of bending the sensingportion 220 in such a way that an electrical response will be producedin that portion. Indeed, as explained further below, the twomechanically coupled but oppositely polarized sensing portions 210 and220 can act together to sense any arriving signal. Moreover, thosevector components that are less likely to be sensed by the sensingportion 210 are more likely to be sensed by the sensing portion 220.

Some embodiments have two polarized sensing portions where thepolarization directions of the sensing portions are not orthogonal, butare non-parallel, having a relative angle of anywhere between zero andninety degrees. Sensing portions that are not polarized parallel to eachother can be used to sense incoming signals from multiple directions.Furthermore, the relative angle can be chosen to provide the sensor withdirection-identification capabilities, or with more efficient magnitudesensing capabilities.

With further reference to FIG. 3, the two sensing layers 210 and 220overlap by an overlap distance 230. In some embodiments, the overlapdistance 230 is approximately 3 mm. The overlap between the sensinglayers 210 and 220 provides an electrical connection, as discussedabove, as well as a mechanical connection. The combination of theoverlap distance 230 and the similar properties of the two sensingportions 210 and 220 can result in the coupled system approximating thebehavior of a single plane. For example, the physical dimensions (widththickness, etc.) and characteristics (stiffness, elasticity, tensilestrength, etc.) of the two sensing portions 210 and 220 are typicallysimilar, because in some embodiments the two sensing portions have beencut from the same type of material. For example, the two sensingportions 210 and 220 can be cut using a template from a single sheet ofstock PVDF. In some embodiments, the two sensing portions 210 and 220have the same dimensions but are cut in different orientations withrespect to the polarization of the stock PVDF. Furthermore, thethickness “d” of the two sensing portions 210 and 220 is typically smallcompared to the other dimensions of the sensing portions 210 and 220,and the dimensions of the combined system are many times greater thaneither the thickness “d” or the overlap distance 230. Some preferredembodiments have sensing portions, each having following dimensions:14.2 mm×30 mm and a thickness “d” of approximately 28 μm.

Preferably, the two sensing portions 210 and 220 have enough overlap 230to remain mechanically coupled and electrically linked, but not so muchoverlap that the resilience of the planar system is significantlyaltered. Thus, the two sensing layers 210 and 220 can physically bendand respond much the same way a continuous plane of the same materialwould respond to an impinging acoustic signal. Some preferredembodiments have an overlap distance 230 of 3 mm. For example, when thesensing portions are 14.2 mm×30 mm, the overlap 230 can occur along the30 mm length of the two sensing portions 210 and 220. In thisconfiguration, the total area of the sensor can be approximately 762mm².

The illustrated configuration also has the advantage of allowingimpinging acoustic signals to arrive at the two sensing layers 210 and220 essentially in unison—that is, such that the time difference ofarrival (TDOA) is minimal. Thus, in some embodiments, configurationsdescribed herein can be referred to as “iso surface optimal materialadherent compliant,” or “ISOMAC” sensors. The two sensing layers withoptimized areas can lie in the same plane, thus generally presenting an“iso surface,” or a surface at which various points lie generally at thesame distance from the source of the impinging acoustic signal.Moreover, as described further below, the materials from which a sensoris constructed can be compliant to the skin surface, bending in responseto an impinging acoustic signal, while at the same time adhering to thesurface of the skin to allow efficient mechanical coupling.

In the illustrated embodiment, the electrical lead 242 is in electricalcontact with the conductive layer 212. The electrical lead 244 is incontact with both the conductive layers 214 and 222. The electrical lead246 is in contact with conductive layer 224. In some embodiments, theelectrical leads 242, 244, and 246 can comprise metal lugs, each havinga 5 mm lip. Other ways of making electrical connections can also beused. As illustrated, the leads 242, 244, and 246 each attach to ashielded pair of twisted wires 248. Because each pair is similar in theillustrated embodiment, each pair of wires has been labeled 248 inFIG. 1. One way to connect the leads 242, 244, and 246 to the wires 248is by soldering or crimping. Electrical connections can also be formedusing an EC adhesive.

Electrical lead 242 corresponds to the A terminal, electrical lead 244corresponds to the C terminal, and electrical lead 246 corresponds tothe B terminal. A and B can be positive terminals, while C is a“common,” or ground terminal. Alternatively, A can be a positiveterminal, while C is a ground terminal, and B is a negative terminal.Various electrical connections can be made to measure the voltagedifference across the sensing layers 210 and 220.

The electrical leads 242, 244, and 246 provide a way for the charge thataccumulates on the conductive layers 212, 214, 222, and 224 to bemeasured using an electrical circuit and connections that will bedescribed further below. If the physical properties of the sensingportions 210 and 220 are known, the equations and physical relationsdescribed herein can allow for the calculation of the acoustic signal'smagnitude, direction, etc. The charge that accumulates on the top andbottom surfaces of the piezoelectric portions encounters relativelylittle resistance in the conductive layers 212 and 214, 222, and 224.Thus, the charge present on any particular layer can be measured atand/or collected from any contact point on that layer.

In a preferred embodiment, the sensing portions 210 and 220 can bemodeled using the physical electrostatic equations for two-platecapacitors. For example, in the illustrated configuration, the twosensing portions 210 and 220 have equal area. This configuration isgenerally analogous to two displaced capacitors with opposite electricalorientations. Assuming that the conductive layers 214 and 222 that arein contact have a negative charge, the capacitance and voltage of thetwo elements individually can be described thus, after charge hasaccumulated:C _(j)=(A _(j) ε)/d _(j) . . . for j=1,2,   (1)where C_(j) is the capacitance of the ‘j’th element, A_(j) is the areaof the ‘j’th element, ε is the electrical permittivity of the medium(e.g., PVDF), and d_(j) is the separation between the conductive layersof the ‘j’th element (which corresponds to the widths of thepiezoelectric portions 216 and 226), or the separation distance betweenthe parallel plates of the capacitor. In a preferred embodiment, d₁=d₂=28 μm. Thus the corresponding voltage signal generated at the twoplates of any element isV _(j) =Q _(j) /C _(j) . . . for j=1,2   (2)

Assuming that the conductive layers 212, 214, 222, and 224 each haveequal area, (A₁=A₂), that each piezoelectric portion 216 and 226 havethe same thickness, (d is constant), and that the piezoelectric portionsare formed from the same material (and thus are equal in electricalpermittivity), (ε is constant), it follows from equations (1) and (2)above thatC_(i)=C₂=C   (3)

Thus, using the capacitance equation for charge ‘Q’ in coulombs given byQ=CV   (4)it follows thatV₁=V₂=V   (5)

Thus, under the assumptions outlined above, one can optimize thephysical configuration as needed. For example, multiple elements can bearranged or the surface area of the elements can be expanded to increasethe voltage output of the sensor. That is, if 68 and d are known andheld constant, A can be varied or optimized in order to optimize ormaximize Q and V. Alternatively, A, d, j, and/or ε can be varied inorder to achieve a desired Q or V. One of the characteristics of some ofthe embodiments described herein is a design where the sensor is sizedto be placed on the skin over the intercostals muscles withoutsignificantly overlapping the ribs. The sensor can also be designed tofit within an adhesive envelope of a given size. One such enveloperequires a sensor to be less than 1 inch×1 inch, for example. Thus, thesensor area may have a certain maximum value. Voltage output can also beengineered to fall within a certain range under any constraints ofelectronic hardware. For example, in some embodiments, preferred voltageoutput is between approximately 0 and approximately 5 volts. The desiredgain, dynamic range, and other characteristics of the electronics intowhich the voltage signal will travel can all provide design parameters.Some embodiments achieve adequate signal strength under these parameterswith a total sensor area of approximately 762 mm², for example.

FIG. 4 depicts the structure of FIG. 2 after more materials have beenadded in layers. As shown, the sensing layers 210 and 220 are positionedbetween metallic shielding layers 410 and 413 (that can, for example, beformed from a metal such as aluminum). The layers 410 and 413 have beenattached to the sensing portions 210 and 220 using layers of adhesivematerial 411 and 412. The adhesive material that forms the layers 411and 412 is preferably a flexible material that is not electricallyconductive. An electrical connection can be made between both of themetallic shielding layers 410 and 413, and terminal C (the common, orground terminal). To make such a connection, a flexible, electricallyconductive adhesive can be used. A flexible adhesive material 409 and414 that is not electrically conductive can be used to attach twoelectrically insulating compliant membranes 415 and 417, as illustrated.One material that can be used to form the compliant membranes 415 and417 is silicone. A layer of biocompatible adhesive 416 can be placed onone of the compliant membranes 415 or 417. In the illustratedembodiment, the biocompatible adhesive 416 has been placed on thecompliant membrane 415. One material that can be used to form thebiocompatible adhesive is “Ludlow Hydrogel,” available from Ludlow, adivision of Tyco Healthcare Group LP, Chicopee, Mass., 01022.

The metallic shielding layers 410 and 413 can provide an electricalshielding effect to minimize unwanted electrical noise. Thus, they canform a continuous or substantially continuous conducting surface thatprevents stray electrical charges from penetrating inside the shieldinglayers 410 and 413. In some embodiments, the metallic shielding layerscan be formed from discontinuous mesh that provides shielding. Theshielding layers 410 and 413 preferably flex with the other layers,allowing the acoustic signal to freely deform the sensor 510 (see FIG.5). Thus, the shielding layers 410 and 413 preferably provide a faradaycage to electrically isolate the sensing portions 210 and 220, while atthe same time not unduly stiffening the sensor or having a decisiveinfluence on its over-all mechanical impedance. However, the stiffnessand resiliency of the shielding layers 410 and 413 can be selected toprovide a portion of the mechanical impedance such that the overallimpedance matches that of the surface of human skin, for example.

The flexible, electrically non-conductive adhesive material that formsthe layers 411 and 412 preferably provides a permanent connectionbetween the sensing portions 210 and 220 and the shielding layers 410and 413. The layers 411 and 412 preferably flex readily when acousticsignals impinge on the sensor 510, operating to mechanically couple thelayers without contributing to the electrical response. The layers 411and 412 also preferably insure that no charge passes from the sensinglayers 210 and 220 to the shielding layers 410 and 413. The layers 411and 412 are preferably uniformly distributed, having very fewirregularities or discontinuities. Furthermore, the layers 411 and 412preferably adhere smoothly and evenly to the surfaces they contact.

The compliant membranes 415 and 417 can provide a protective, waterrepellant layer that protects the electrical connections inside thecompliant membranes 415 and 417 from unwanted moisture. In someembodiments, the compliant membranes 415 and 417 form a continuous outerlayer that surrounds all other layers except the biocompatible adhesive416. The compliant membranes 415 and 417 can also have a mechanicalimpedance that corresponds to that of human skin, for example. Thecompliant membranes 415 and 417 can thus continuously conform to thechanging contours of the surface of human skin as the skin responds toimpinging acoustic energy. The compliant membranes 415 and 417 help keepthe acoustical loss between the skin and the sensor at a minimum. Thedescribed configuration can provide for good sensor sensitivity by usinga silicone compliant material to interface with the skin surface.

The flexible, electrically non-conductive adhesive material that formsthe layers 409 and 414 preferably provides a permanent connectionbetween the shielding layers 410 and 413, and the electricallyinsulating compliant membranes 415 and 417. The layers 409 and 414preferably flex readily when acoustic signals impinge on the sensor 510,operating to mechanically couple the layers without contributing to theelectrical response. The layers 409 and 414 also preferably help insurethat no charge passes between the outside of the sensor 510 and theshielding layers 410 and 413. The layers 409 and 414 are preferablyuniformly distributed, having very few irregularities ordiscontinuities. Furthermore, the layers 409 and 414 preferably adheresmoothly and evenly to the surfaces they contact.

In some embodiments, a biocompatible adhesive 416 is used to improve themechanical and acoustic connection between the compliant membrane 415and skin. The biocompatible adhesive 416 can be “Hydrogel,” (aspreviously described), which can be positioned at the skin-sensorinterface to improve sensitivity and acoustic/mechanical coupling. Insome embodiments, the biocompatible adhesive 416 is smeared onto thehuman skin surface where the sensor 510 will be placed, and the sensor510 is pressed onto the same area of the skin. The adhesive 416 can alsobe placed on the sensor 510 before it is pressed into place. In someembodiments, the biocompatible adhesive 416 is located beneath aremovable strip (not shown) on the sensor when the sensor 510 ispackaged, and the user can remove the strip to reveal the biocompatibleadhesive 416 underneath, immediately prior to using the sensor 510.

As schematically illustrated in FIG. 5, the layers described above canform a sensor 510 and leads that correspond to terminals A, C, and B, asdescribed above. As illustrated, FIG. 4 shows a schematic,cross-sectional view of the sensor 510 taken along lines “4-4.”

With reference to FIG. 6, the described sensors can be advantageouslyemployed in a system for detecting and processing heart sounds, such asthat described in U.S. patent application Ser. No. 10/830,719, filedApr. 23, 2004, and published on Feb. 17, 2005, the entirety of which ishereby incorporated by reference and made part of this specification.The improved sensing abilities of the sensors 510 can help enable theaccurate detection and localization of stenoses in portions of a humanheart, for example. In some embodiments, multiple sensors can beemployed to collect data from a plurality of locations on a human bodysurrounding a human heart, for example. In particular, FIG. 6schematically shows one possible configuration of multiple sensors 510and their general placement on a human body. In some embodiments, foursensors can be positioned on the surface of the skin, generally on theexternal anatomy surrounding the human heart. The sensors 510 cancollect acoustic data from sounds emanating from within the body (e.g.,the coronary artery). For example, the sensors 510 can gather data forthe same acoustic signal from multiple spatial points. The sensors canelectronically communicate with a signal processing system 612,conveying, for example, electrical signals that convey informationrelating to acoustic signals. An example of such a signal processingsystem is described in U.S. patent application Ser. No. 10/830,719. Thedescribed sensors can aid in the clinical benefits described byproviding accurate acoustic data, whatever the arrival direction of theacoustic signal.

With reference to FIG. 7A, a polarized piezoelectric slab 710 isillustrated. The slab 710 undergoes a strain in the polarizationdirection, resulting in an accumulation of charge at the surfaces of theslab. The piezoelectric material at the top of the slab 710 undergoestensile forces 711 in the polarization direction, which results in anaccumulation of positive charge at the upper surface. In contrast, thepiezoelectric material at the bottom of the slab 710 undergoescontractive forces 713 in the polarization direction, which results inan accumulation of negative charge at the lower surface.

FIG. 7B illustrates schematically how a polarized piezoelectric slab 720that undergoes a strain in a direction orthogonal to its polarizationdoes not accumulate significant charge at its surfaces. Thus, the singlepolarized piezoelectric slab 720 does not normally sense inputeffectively if that input does not cause a strain that is aligned withpolarization of the slab 720. The piezoelectric material at the top ofthe slab 720 is subjected to the same tensile forces as the slab 710illustrated in FIG. 7A, and the piezoelectric material at the bottom ofthe slab 720 undergoes the same contractive forces as the slab 710.However, because the slab 720 is polarized orthogonally, nopiezoelectric effect is produced in the slab 720.

In contrast, FIG. 7C illustrates how the same polarized piezoelectricslab 720 can accumulate charge, and thus effectively “sense” a signalthat causes a differently oriented force on the slab 720. In particular,the piezoelectric material at the top of the slab 720 undergoes tensileforces 721 in the same direction of polarization, which results in anaccumulation of positive charge 722 at the upper surface. In addition,the piezoelectric material at the bottom of the slab 720 undergoescontractive forces 723 in the same direction of polarization, whichresults in an accumulation of negative charge 724 at the lower surface.

FIG. 8A illustrates a two dimensional vector 830. The vector 830 has amagnitude (corresponding to its length) and a direction in the plane ofthe page. As illustrated, the vector 830 can be “resolved” into twovector components, the vertical component 832 and the horizontalcomponent 834. The combination of the two components 832 and 834 canprovide the same information inherent in the original vector 830. Thevertical component 832 and the horizontal component 834 can also becharacterized as a “basis set” onto which the vector 830 can beexpanded.

Just as this vector 830 can be resolved into components, an incomingsignal can be represented as a vector quantity that can be resolved intotwo components in a Cartesian coordinate system (or another basis set).This concept can be employed to combine two orthogonally polarized (ornon-parallel) sensing portions in a single sensor, and from therespective signals of the two sensing portions, directional componentscan be calculated and an approximation for the signal magnitude can beproduced. Thus, if the polarized slabs 710 and 720 are physicallycombined such that one slab is polarized in a direction that isorthogonal (or non parallel) to the other slab, a device that sensessignals coming from any direction can be constructed. Such a devicepreferably is configured to allow the impinging acoustic signals toarrive at the two slabs essentially in unison—that is, such that thetime difference of arrival (TDOA) is minimal. This minimization of theTDOA can be achieved when the sensor comprises two portions of thin PVDFmaterial that partially overlap as illustrated herein.

FIG. 8B illustrates a piezoelectric slab 810 that is undergoing strainthat is not aligned with the slab's polarization axis. That is, thestrain is in a direction “S” that is neither parallel to norperpendicular to the polarization axis “P.” The strain “S” can berepresented in terms of some orthogonal components 812 (components thatare perpendicular to “P”) and some parallel components 814 (componentsthat are parallel to “P”). Under the influence of these the varioustensile and contractive forces that result from the strain “S,” the slab810 does not sense the orthogonal components 812 of these forces aseffectively as it senses the parallel components 814. This kind ofunaligned strain is typical of that generated by an acoustic signal thatoriginates in the body and arrives at the surface of a patient's skin.Indeed, because of the difficulty in predicting the arrival direction ofany acoustic signal to be measured (especially when such a direction maybe part of the information sought from the test in the first place), itis unlikely that a sensor's polarity can be perfectly aligned with anincoming acoustic signal. Furthermore, if information is sought relatingto a signal's magnitude, but the direction of the signal does not needto be known, a combined system can be sensitive to signals from anydirection without a need for aiming the sensor. If the two slabs 710 and720, in the orientations illustrated in FIG. 7, are combined to form amechanically coupled system that undergoes the strain illustrated inFIG. 8B, the two sensing portions 710 and 720 can each detect vectorcomponents of the acoustic signal. Furthermore, if the two sensingportions 710 and 720 are located in generally the same plane, the TDOAwill be minimized and the two sensing portions will be responding toessentially the same signal. Thus, the configuration of sensing portions210 and 220 illustrated in FIGS. 2-4 can provide multi-directionalsensing capability and minimize sensing errors due to a large TDOA.

With reference to FIG. 9A, an acoustic source 910 is schematicallyillustrated, and a circular wavefront 914 is shown emanating from thesource 910. Another wavefront 918 is illustrated further away from thesource 910, as energy travels upward toward a surface 920. In someembodiments, the source 910 can be located within a human body, and thesurface 920 can be the human skin on the surface of the body. A sensor510 is schematically illustrated, showing sensing portions 210 and 220within the sensor 510. Before the wavefront 918 arrives at the surface920, the sensor 510 is not under any significant strain and rests in anequilibrium, essentially zero-signal configuration.

With reference to FIG. 9B, the wavefront 918 has arrived at and deformedthe surface 920, which is protruding outward, causing a strain in thesurface 920. The sensor 510 is mechanically coupled to the surface 920and undergoes a proportional strain when the wavefront 918(corresponding to an acoustic signal) arrives at the surface 920. Thestrain causes charge to accumulate on the surfaces of the sensingportions 210 and 220, which can in turn cause a signal to be transmittedfrom the sensor 510 containing information about the magnitude anddirection of the wavefront 918.

FIG. 9C schematically illustrates a three-dimensional view of thearrival of the wavefront 918 at the surface 920 depicted in FIG. 9B. Asillustrated, the sensing portions 210 and 220 undergo strain in morethan one dimension. Thus, the sensing portions 210 and 220 can bepolarized in orthogonal directions and the system can gather effectivedirectional information, as described above. This illustrates oneadvantage of the described sensor embodiments, namely their ability todetect signals from multiple directions. The described characteristicscan enhance the sensors' ability to convert acoustical to mechanicalenergy from acoustic waves that cause of the skin to bend. For example,when it is mounted on human skin, the sensor 510 can receive signalsfrom any direction in the 360° field of view.

FIG. 10 schematically illustrates an embodiment of two sensing portions1010 and 1020. The upper sensing portion 1010 has an upper conductivelayer 1012 that contacts a lead A, and a lower sensing portion 1014 thatis in electrical communication with a lead C. The upper conductive layer1022 of the lower sensing portion 1020 is also in contact with the leadC, and the lower conductive layer 1024 of the lower sensing portion 1020is in contact with a lead B. The upper sensing portion 1010 can bepositioned to partially overlap the lower sensing portion 1020, asshown. Preferably, the area of the upper and lower sensing portions ofany given sensor embodiment are approximately equal. Equal areassimplify the capacitance calculations because the two areas can becancelled out by dividing both sides of the equation by the area. Thus,in the embodiment illustrated in FIG. 10, the area of the upper sensingportion 1010 is approximately equal to the area of the lower sensingportion 1020. Furthermore, the conductive layers 1012 and 1024 arepreferably polarized in one direction, and the conductive layers 1014and 1022 are preferably polarized in an orthogonal direction, as shownby the polarization arrows labeled “P” in FIG. 10.

FIG. 11A schematically illustrates a perspective view of an embodimentof two sensing portions, an outer ring 1110 and an inner ring 1120.Preferably, the two rings have the same area, though they do not havethe same radius. The outer ring 1110 has an upper conductive layer 1112that electrically contacts the bottom conductive layer 1124 of the innerring 1120 through a contact strip 1132. This configuration allows thetwo sensing portions 1110 and 1120 to be in electrical contact withouthaving any overlapping portions. Furthermore, in this embodiment, thetwo sensing portions 1110 and 1120 are approximately coplanar. Aterminal A is shown to be in electrical contact with the upperconductive layer 1122 of the inner ring 1120. A terminal B is shown tobe in electrical contact with the lower conductive layer 1114 of theouter ring 1110. A terminal C is shown to be in electrical contact withthe upper conductive layer 1112 of the outer ring 1110, and by extensionwith the lower conductive layer 1124 of the inner ring 1120.

FIG. 11B schematically illustrates a plan view of the embodiment of FIG.11A. FIG. 11C schematically illustrates a cross-sectional view takenalong the lines 11C-11C of FIG. 11B. The approximately coplanar natureof this embodiment is apparent in the cross-sectional view of FIG. 11C.Furthermore, the conductive layers 1112 and 1124 are preferablypolarized in one direction, and the conductive layers 1114 and 1122 arepreferably polarized in an orthogonal direction, as shown by thepolarization arrows labeled “P” in FIG. 11. The surface areas of theinner and outer rings are preferably equal, which can allow the sensorto achieve a constant gain or signal response when acoustical signalsarrive from any direction. This configuration can provide especiallyefficient data when sensing a spherical wave front because of itsgeneral cylindrical symmetry.

FIG. 12A schematically illustrates an embodiment of an electricalcircuit that can be used to represent the electrical response propertiesof the embodiments discussed above. A terminal A is connected to avariable voltage source V_(S2). The variable voltage source V_(S2) cancorrespond to the conductive layers 212, 1012, or 1122, for example. Aterminal B is connected to a variable voltage source V_(S1). Thevariable voltage source V_(S1) can correspond to the conductive layers224, 1024, or 1114, for example. The conductive layers have variablevoltages that depend on the amount of strain on (and charge thataccumulates on the surfaces of) the piezoelectric portions with whichthey are in electrical contact. A terminal C is connects the oppositesides of the two variable voltage sources—through resistances R_(S1) andRS₂—to ground. In practical effect, the resistances R_(S1) and R_(S2)generally approach zero. The C terminal can correspond to the conductivelayers 214 and 222, 1014 and 1022, and 1112 and 1124, for example. Whenthese layers are connected to ground, they can draw (or deposit) as muchcharge as needed to balance out the charge that flows to terminals A andB as a result of the piezoelectric effect on the sensing portions. Thevoltage difference across terminals A and C is measured, and the voltagedifference across terminals C and B is measured. In some embodiments,the A-C voltage can provide information relating to the signalcorresponding to one vector component of the impinging acoustic signal,and the B-C voltage provides information relating to the other vectorcomponent.

FIG. 12B schematically illustrates an embodiment of an electricalcircuit that can be used to test the circuit illustrated in FIG. 12Aand/or analyze the data produced by the embodiments described above. Theterminal a can be connected to A, the terminal c can be connected to C,and the terminal b can be connected to B. If the switch 1222 is closed,a multimeter 1220 can measure the current, resistance, and/or voltagedrop across terminals A and C. If the switch 1224 is closed, themultimeter 1220 can measure the current, resistance, and/or voltage dropacross terminals C and B. Similarly, if the switches 1226 and 1228 areboth closed, the signal processor 612 can process the signals providedby the sensor to determine electrical and acoustic characteristics ofthe sensed system.

The processes described herein can advantageously be adjusted forefficient manufacture. For example, electrical connections and circuitscan be formed using chemical deposition and integrated circuitprocesses. Moreover, materials can be deposited, one onto another, in aform and using a deposition process that eliminates the need for theadhesive materials described herein.

Although the present inventions have been described in terms of certainpreferred embodiments, various features of separate embodiments can becombined to form additional embodiments not expressly described.Moreover, other embodiments apparent to those of ordinary skill in theart after reading this disclosure are also are within the scope of theseinventions. Furthermore, not all of the features, aspects and advantagesare necessarily required to practice the present inventions. Thus, whilethe above detailed description has shown, described, and pointed outnovel features of the invention as applied to various embodiments, itwill be understood that various omissions, substitutions, and changes inthe form and details of the device or process illustrated may be made bythose of ordinary skill in the technology without departing from thespirit of the invention. The inventions may be embodied in otherspecific forms not explicitly described herein. The embodimentsdescribed above are to be considered in all respects as illustrativeonly and not restrictive in any manner. Thus, scope of the invention isindicated by the following claims rather than by the foregoingdescription.

1. A method of manufacturing an acoustic sensor comprising: providing afirst piezoelectric layer having a first polarization axis; providingtwo conductive layers, one on either side of the first piezoelectriclayer; providing a second piezoelectric layer having a secondpolarization axis and two conducting layers, one on either side of thesecond piezoelectric layer, the second piezoelectric layer positionedgenerally coplanar to the first piezoelectric layer, one conductinglayer of the first piezoelectric sensing portion in electrical contactwith one conducting layer of the second piezoelectric sensing portion,the first and second polarization axes having a non-zero angle betweenthem.
 2. The method of claim 1, further comprising partially displacingthe first piezoelectric layer from the second piezoelectric layer. 3.The method of claim 1, further comprising forming the electrical contactbetween one conducting layer of the first piezoelectric layer and oneconducting layer of the second piezoelectric layer by directlycontacting the two conducting layers.
 4. The method of claim 1, furthercomprising forming the electrical contact between one conducting layerof the first piezoelectric layer and one conducting layer of the secondpiezoelectric layer with a bendable conductive lead between the twoconducting layers.
 5. The method of claim 1, further comprisinginverting the first and second piezoelectric layers with respect to eachother such that a top conducting layer of one piezoelectric layerreceives charge of the same polarity as the bottom conducting layer ofthe other piezoelectric layer when the two piezoelectric layersexperience similarly-oriented strain.
 6. The method of claim 1, furthercomprising providing at least one piezoelectric layer comprisingpolyvinylidene fluoride.
 7. The method of claim 1, further comprisingproviding metal conducting layers.
 8. The method of claim 1, furthercomprising providing a piezoelectric layer that is thicker than the twoconducting layers combined.
 9. The method of claim 1, further comprisingorienting the first and second polarization axes at an angle ofapproximately ninety degrees.